1. The Field of the Invention
The present invention is related to optical devices. In particular, the present invention relates to a fiber optic coupler device for dense wavelength division multiplexing applications.
2. The Relevant Technology
Narrowband optical filters are employed widely in the optical communications industry. One important function of such filters is in wavelength division multiplexing (WDM) systems, where they are used to combine (multiplex) or segregate (demultiplex) wavelengths in a multi-wavelength optical communication stream. A WDM system which employs optical carriers having a frequency spacing of 800 gigahertz or less is called a dense wavelength division multiplexing (DWDM) system. WDM and DWDM systems are most commonly employed in a narrow region of the near infrared optical spectrum (nominally 1500 nanometers to 1600 nanometers), where silica-based optical fiber has especially low loss, and erbium-doped fiber amplifiers (EDFAs) may be employed to periodically boost optical signals.
Optical filters which satisfy the spectral combination and segregation requirements for WDM and DWDM systems must transmit a very narrow band of light while rejecting light at nearby wavelengths. One type of DWDM system for the 1550 nanometer waveband which is becoming common employs a channel spacing of 100 gigahertz; this translates into a requirement for filters which transmit a band of light approximately .+-.0.2 nanometers while efficiently rejecting light .+-.0.6 nanometers from the center wavelength of the filter. Such filters must be centered precisely at a predetermined wavelength. This combination of requirements makes optical filters for DWDM applications difficult to build and consequently relatively expensive to manufacture.
One known technique for providing a selective narrow band optical filter is by utilizing a wavelength selective interference filter element whose wavelength characteristic depends on the angle of incidence. Thus, by varying the angle of light incident upon the interference filter, the wavelength of the light that is transmitted by the filter varies. Often, these filter elements are used in free space configurations, wherein a beam of light exiting an optical fiber or other waveguide is directed through free space into a wavelength selective interference filter element at a predetermined angle. Such configurations can be problematic. For example, positioning and affixing an optical fiber in a predetermined position and at a predetermined angle relative to a filter element can be difficult to accomplish.
Optical coupler devices have been developed which utilize two collimating gradient index (GRIN) lenses coaxially arranged with an optical interference filter disposed therebetween. Two or more ports are formed on the end faces of the coupler devices by attaching optical fibers thereto. Such a coupler device is disclosed in U.S. Pat. No. 5,612,824 to Si et al., the disclosure of which is incorporated herein by reference.
Conventional optical coupler devices can spatially segregate one or more International Telecommunications Union (ITU) wavelength channels from a multichannel optical communication stream. Such coupler devices may also be employed in a complementary configuration to combine one or more channels with a multichannel optical communication stream. During manufacture, the center wavelength of the passband of such a coupler may be adjusted by laterally translating the input and output fibers to off-axis positions along the conjugate surfaces of the collimating lenses. This results in an increased angle of incidence at the interference filter, with the filter passband shifting to a shorter wavelength. This adjustment capability relaxes the wavelength centering requirement on the filter, thereby improving the manufacturability of the coupler.
Optical couplers have been successfully deployed in a wide variety of optical systems including four-channel DWDM systems with a passband width on the order of 4 nm, and in 200 gigahertz (GHz) DWDM systems with a passband width on the order of 1 nm. A commercially available three-port coupler which has been utilized in DWDM systems is available from JDS Fitel Inc. of Nepean, Canada.
As the demand for telecommunication bandwidth has grown, so has the need for increasing the number of optical wavelengths in DWDM systems. The limited gain bandwidth of optical amplifiers translates into a need for carriers or channels more closely spaced in wavelength. Consequently, the state-of-the-art in interference filter based DWDM systems is moving from 200 to 100 GHz channel spacing. In order to transport as much information as possible through a given fiber, a 100 GlIz filter must exhibit a high transmittance (T) level in the passband (T&gt;90%), very narrow passband width (e.g., T&gt;90% for 1549.8 nm&lt;.lambda.&lt;1550.2 nm), and very steep cuton and cutoff slopes adjacent to the passband (e.g., T&lt;0.1% for .lambda.&gt;1550.6 nm and for .lambda.&lt;1549.4 nm). It is anticipated that even narrower passband widths will be required in future systems.
Existing co-axial GRIN lens coupler designs, when used with 200 and 100 GHz channel spacings, exhibit an anomalous reduction in peak transmittance (i.e., increased insertion loss). This problem increases dramatically as the filter passband width decreases and as the angle of incidence at the filter increases. This phenomenon will be referred to hereafter as angle dependent loss.
Accordingly, there is a need for improved optical devices which overcome or avoid the above difficulty.